Dental caries represents one of the most common chronic diseases affecting young children, and is a multi-factorial disease involving complex interactions of risk factors. The mutans streptococci (MS) group is among the most cariogenic microorganisms associated with dental caries. We propose to identify the microbial factors affecting the genetic diversity and potential selection of MS strains following caries restorative therapy in children. In prior work, we described genetic fingerprints of MS isolates (N=828) collected before and after treatment from children exhibiting severe early childhood caries (S-ECC). We have also identified the dominant MS genetic strains that occur in carious and non-carious sites within the oral cavity of individual patients (N=20), and have characterized strain cariogenicity properties. Our published studies have identified 39 MS genetic strains, and have found that caries restorative therapy in some patients results in population shifts to highly acidogenic or acid-tolerant MS strains, with single dominant MS strains appearing at 1-year post-therapy. The objectives of this current project are to initiate a new cohort of 250 patients where restorative and adjunctive therapy measures can be applied independently to ascertain selective effects, determine if MS genetic strains with distinct cariogenicity profiles undergo preferred selection in carious sites versus non-carious sites within the oral cavity of individual patients, and develop a standardized genetic database of MS variants with defined cariogenic phenotypes that may serve as predictive microbial identifiers for dental caries and treatment outcomes. The study hypothesis is that children with S-ECC may possess distinct genetic strains of MS, which undergo resistance and/or selection during specific aspects of caries restorative treatment, and this impact the effectiveness and long-term outcome of the treatment. The translational goal of this project will be to provide insight on the efficacy of restorative an adjunctive therapy practices in the elimination and/or reduction of certain MS genetic strains in caries-active children. The significance of this study is profound, as treatment in some patients with S-ECC results in the appearance of dominant and highly-cariogenic MS genetic strains, which conceivably could place the patient at even higher risk. This study is innovative because of its potential to predict the outcome of caries preventive therapy based on the identification of the MS genetic strains. The OHSU School of Dentistry is an eligible institution for an R15 AREA award. The Principal Investigator and collaborative faculty group have mentored over 13 graduate dental residents, 30 dental students and 18 undergraduate students in research over the past 5 years, and we anticipate continued success in the recruitment of new student researchers for the R15 award.